The present invention relates generally to reflected signal ranging systems and, more particularly, to an electromagnetic ranging system employing a hybrid of pulsed and continuous wave techniques to achieve very high ranging accuracy.
Present day radar techniques for determining the distance or range to a target have serious shortcomings when extremely high accuracy is required. The inability to give repeatable and highly accurate results has heretofore meant that radar ranging techniques have been ruled out for whole classes of applications which could be conveniently handled using radar techniques, if the accuracy could be improved.
As an example, consider the problem of measuring water height in a river. The classical solution is to place a measuring stick in the river and periodically check it to see where the water line crosses the stick. The classical technique requires a person to periodically check the water height, making this solution infeasible for large scale geodetic surveys. If radar techniques could achieve the desired accuracy, then water height could be quite easily measured by placing a radar transceiver on a bridge or other fixed structure and by measuring the water height as a function of range between the transceiver and the water's surface. The data could then be broadcast to a nearby geodetic station for effectively continuous monitoring of the water height. Regretfully, present day radar is not accurate enough to measure distances on the order of inches or fractions of inches. Hence, radar techniques have not been available to solve the exemplary water height problem.
The present invention provides a precision ranging system using radar techniques which achieves accuracies never before achieved with conventional radar techniques. The invention employs a combination of short pulse ranging and continuous wave processing techniques to measure the range of a noncooperative target with an absolute accuracy of less than plus or minus 0.015 feet or better. According to the invention, a short pulse or burst of electromagnetic energy is transmitted and the return from the target is envelope detected. The magnitude of the return pulse is sensed and fed to a level controller in the transmitter to ensure that the return pulse maintains a constant amplitude. The return pulse is then threshold detected and converted to a square wave which is in turn down converted. The phase of the down converted signal is then measured and yields the range under measurement.
By ensuring that the amplitude of the received pulse remains constant, the invention ensures that the slope of the leading edge of each received burst is constant. Thus, by maintaining the slope constant, the threshold crossing point is held in a constant relationship relative to the transmitted pulse, yielding extremely high accuracy. During the down conversion process, the phase of the return signal is preserved while the time scale is expanded by several orders of magnitude for much greater precision. The invention is virtually immune from range errors due to returns from distributed clutter (such as rain, fog or aerosols) or from objects in the detection pattern side lobes or from multipath distortion. The invention is not limited to any specific portion of the electromagnetic spectrum and can be used with either single or dual antenna (optic) transceivers and with direct or heterodyne detection schemes.
According to the invention, a hybrid technique is employed which achieves the advantage of short pulse, leading edge detection ranging systems in avoiding the clutter and side lobe problems, and also achieves the advantages of continuous wave ranging systems which are not limited in absolute accuracy by currently achievable rise times. Accordingly, a method is provided for determining the range to a target comprising the steps of reflecting a first electromagnetic signal from the target, receiving the first signal and generating a second electromagnetic signal at an energy level determined by the strength of the first received signal. The method continues by reflecting a second signal from the target, receiving the second signal and determining a phase difference between the generated second signal and the second received signal. Finally, the range to the target is determined in accordance with the phase difference. Further, in accordance with the inventive method, the second received signal is down converted by mixing or multiplying with a reference signal of slightly different frequency to produce sum and difference signals, (the difference signal being at a much lower frequency than the carrier). The difference signal, so produced, is used in determining the phase difference.
The invention further provides an apparatus for determining the range to a target comprising a transmitter having a means for providing oscillations at a controlled amplitude and a gating means for modulating the oscillations. The invention further comprises a receiver having means for providing a control signal based on the signal strength of received signals, for controlling the amplitude of the oscillations produced by the transmitter. The receiver further comprises a threshold detecting circuit and a down converter means for providing a shifted frequency signal having a phase portion. The receiver further comprises a means for comparing the phase portion of the shifted signal with the phase portion of a reference signal and for thereby determining the range to the target.
For a more complete understanding of the invention, reference may be had to the following specification and to the accompanying drawings.